[vtkusers] How does "Render" work?

[John Biddiscombe]

> When I create a vtkDataSetMapper with input = "vtkUnstructuredGrid" and
the
> UnstructuredGrid doesn't have datas, the function "Render" isn't executed.
> Why?? Where is this verified???

The Renderwindow/Renderer (can't remember which is responsible) does an
Update on all Actors. Each Actor requests its mapper to update. Each mapper
its dataset..etc.

Before calling render on each mapper, the renderer does a check on visible
actors. Any Dataset wiuth no cells will have a bounds of (0,-1,0,-1,0,-1)
(invalied). This actor gets skipped and not rendered. Hence no calls to
internal RenderOpaque, Render etc etc on the mapper.

JB